The present invention relates generally to direct liquid fuel cell systems. More particularly the invention relates to passive fuel delivery and handling for a liquid fuel cell system with a pressure-maintaining fuel cartridge.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Organic fuel cells are a useful alternative in many applications to hydrogen fuel cells, overcoming the difficulties of storing and handling hydrogen gas. In an organic fuel cell, an organic fuel such as methanol is oxidized to carbon dioxide at an anode, while air or oxygen is simultaneously reduced to water at a cathode. Organic/air fuel cells have the advantage of operating with a liquid organic fuel. While methanol and other alcohols are typical fuels of choice for direct feed fuel cells, recent advances presented in U.S. Patent Application Publication Nos. 2003/0198852 (“the '852 publication) and 2004/0114418 (“the '418 publication”) disclose formic acid fuel cells with favorably high power densities and output currents. Exemplary power densities of 15 mW/cm2 and greater were achieved at low operating temperatures, thereby demonstrating the viability of formic acid fuel cells as compact electric power generation devices.
Fuel cell technology is evolving rapidly as an energy supply for portable electronic devices such as laptop computers and cellular telephones. However, mobile devices and other low power applications require a method to substantially continuously supply fuel to the fuel cells, and as well as a method to replenish the fuel once it becomes depleted. A common method for supplying fuel is to encase the fuel in a closed, pressurized cartridge that is removable and replaceable within the electronic device to be powered. It is therefore desirable for the fuel cell to operate at high power densities and for the stored fuel to have a high latent power density. Accordingly, there is a need to be able to store a relatively high concentration of the fuel to be fed to and consumed by the fuel cell(s). For certain vaporizable organic fuels such as formic acid, storing highly concentrated fuel solutions typically results in problematic fuel vaporization during storage and at typical operating temperature ranges. As a result, low concentrations of the vaporizable fuel are typically employed, thereby limiting stored energy density of the fuel to be fed to the fuel cell(s).
Problems also exist with current methods of operating a fuel cell system in which the fuel fed to the fuel cells is delivered from a closed pressurized container during fuel cell operation, and in which the flow of fuel should stop positively when not required for fuel cell operation. Operating such system involves the employment of many system components, thereby increasing the size, volume and complexity of such systems and reduced system efficiencies because of a resulting increase in parasitic power drawn from the system by a multiplicity of system components. System simplification to reduce the number, size, volume and complexity of system components, as well as reduction in the amount of parasitic power drawn from the system, can be accomplished by reducing the number and complexity of active components within the system. Making such a system perform effectively, with minimal components, requires careful integration of system components and functions over a range of operating conditions.
In general, unidirectional flow of fuel from a container with a fuel compressed to moderate pressures cannot deliver fuel to the fuel cell system in an effective manner. As the fuel is discharged from the container, a vacuum would eventually be created within the container, and remaining fuel would become undeliverable. Additionally, fuel recycling is desirable in fuel cell systems in which un-reacted fuel would be wasted if not returned to its storage container. In the case of reactive fuels such as formic acid, the un-reacted fuel and vapor is desired to be contained or converted to benign byproducts for release to the environment.
The present system design incorporates solutions to the foregoing problems of storing, delivering and recovering liquid fuel to be fed to direct liquid feed fuel cells in a low power range suitable for portable electronic devices such as laptop computers and cellular telephones. Unlike direct methanol fuel cells, the present system is designed to accommodate a vaporizable fuel such as an aqueous formic acid solution by providing for the out-gassing of vaporous fuel.
Specifically for fuel delivery from a cartridge, there is a range of solutions to the problems of providing a fuel storage cartridge for delivering fuel to a fuel cell in a low power range suitable for mobile end-uses. These solutions have typically been designed for methanol-based fuel, which in comparison to a liquid fuel such as formic acid fuel, has no requirements for out gassing relief of evaporating vapors, particularly during periods of storage.
Typically, cartridges include housing, a fuel bladder or liner in the housing and a fuel port coupled to the bladder for refueling and fueling. There is a common problem of how to most effectively and efficiently extract or deliver fuel from the cartridge to the fuel cell system while reducing overall system complexity and avoiding additional problems, and increasing effective stored energy density by reducing additional space taken up by the cartridge.
Known solutions belong to the following groups, movable springs, expandable bladders, external or internal powered fuel pumps, wicking fuel ports, and interaction of multiple cavities or bladders.
The most common form of active pumped cartridge employs a movable spring, spring biased plate or wall to push on the liner or bladder and continue to provide pressure as the volume of fuel decreases in the bladder. For example, U.S. Patent Application Publication Nos. 2003/0129464 and 2004/0072049 describe spring and plate mechanisms. U.S. Pat. No. 6,924,054 and PCT/International Publication No. WO 03/043112 describes movable barriers with a spring. Cartridges employing mechanical springs again restrict the space utilization and stored energy density. Further they are mainly suited for end-uses where bladder volume decreases with fuel delivery and a compressive force is required to maintain fuel pressure.
Expandable bladders are disclosed in U.S. Patent Application Publication Nos. 2004/0013927 and 2002/0197522, along with expandable pressure members that provide a positive pressure on the bladder. The expandable bladder disclosed is impermeable to the methanol fuel. An example of the pressure member is compressible foam butted against the bladder. Limitations of this design are (a) that the extra space of the compressible foam limits stored energy density (the volume of the bladder and foam are approximately equal), and (b) that the design is unsuitable for formic acid fuel as the fuel vapor is not managed or relieved.
Actively pumping the fuel out of the cartridge is commonly done, but requires extra components. Pumps can be employed to pump gas back into the cartridge to pressurize the bladder as described in U.S. Patent Application Publication No. 2005/0058858 in which air is pumped back into the cartridge cavity through a second port for maintaining pressure as the bladder volume decreases. Relying only on fuel pumps reduces overall system energy efficiency due to the extra power drain.
A common design for passive fuel delivery is providing wicks coupled between the liner and the fuel inlet, acting by capillary action to transfer fuel. U.S. Pat. No. 6,726,470 and U.S. Patent Application Publication No. 2004/0126643 are representative of wick fuel delivery. Problems with wicking systems include material incompatibility with formic acid fuel, and suitable control of fuel delivery rate. Particularly for low power systems where the fuel dose is small and requires precise control, wicking delivery is not suited.
Multiple cavities or bladders can be employed for pressure management and containing waste fuel. For example, U.S. Patent Application Publication No. 2003/0082427 describes a dual bladder cartridge with one of the bladders having an internal biased spring to pressurize the primary fuel bladder, and two ports for delivering fuel and receiving waste products. The cartridge is additionally complex and costly due to the extra components and less than optimal for storage energy density. In particular the waste product is not reused in this case.
Due to the hazardous characteristic of formic acid, it is a requirement that not more than very low levels of formic acid or vapors are released from the cartridge, known hot swappable liquid fuel cartridges are primarily designed for methanol fuel not formic acid. Methanol fuel storage does not have the same problems of generated gas bubbles that can enter the fuel delivery line and interrupt fuel delivery in various orientations. In particular, there is a no solution for a cartridge and fuel cell system for formic acid that can supply and handle fuel and be operable over a wide range of orientations, without adverse emissions or change in operations.
There is thus a need for a fuel cartridge and matching fuel cell system, that is well-suited to vaporizable liquid fuels such as formic acid, that has a design for pressurizing and delivering vaporizable liquid fuel without powered or movable components, and that is suitable for safely storing formic acid, having a single cavity enclosure for high energy density, recycles depleted fuel from the fuel cell system, and meets safe emissions, and enables an associated fuel cell system to operate with limited movable parts.
A passive-pumping liquid feed fuel cell system comprises:
In operation, when the fuel delivery module flow regulating mechanism is in an open position, the pressurized fuel stream is discharged from the cartridge module, and when the fuel delivery module flow regulating mechanism is in a closed position, the recycle fuel stream is admitted into the cartridge module.
A solution is provided to at least some of the problems previously described, by combining a passively pressurized fuel cartridge having a fuel management port interface with a fuel cell system with closed fuel circulation, the combination managing the resulting unused fuel and vapor byproducts during fuel cell operation. Such a system is particularly advantageous with aqueous formic acid fuel, where the low flashpoint results in vapors at normal storage and operating temperatures, and unused fuel and by-products are unsuited for release into the user environment, particularly for handheld mobile device applications.
Turning to
Fuel Cartridge Module
As shown in
As further illustrated in
The design and operation of a fuel cartridge module 20, which is suited for integration with overall fuel cell power generation system 10 in
Stored formic acid fuel in the bladder 24 will naturally evaporate and the formic acid vapor exits the bladder walls, increasing the cavity pressure. The relief pressure setting is selected to keep the internal cavity pressure within a preferred range. In typical use, there is preferably no gas released outside the cartridge, however in extended storage conditions the pressure can exceed the relief pressure setting. The cavity pressure forms an integral function of the passive fuel cartridge, as it pressurizes the bladder fuel sufficient to deliver fuel through the port 25 to the coupled fuel delivery module and fuel cells. Compression elements 26a are shown on the bladder for additional minimum pressurization of the stored fuel. The fuel cartridge has a desired fuel delivery pressure range as determined by the associated fuel cell design and delivery flow path. For the case of formic acid fuel stored in the illustrated bladder, a preferred example of the maximum of this delivery range is 8 pounds per square inch (55.2 kPa); therefore, the pressure relief valve opens at approximately 8 psi (55.2 kPa) pressure to maintain the internal cavity pressure of 8 psi (55.2 kPa) or less. Typically, the pressure maximum for the case of formic acid fuel is 15 psi (103.4 kPa) or less to eliminate explosion risk. Orientation problems due to mixed gas and liquid within the bladder are thereby overcome by the cartridge and bladder combination. Cartridge 20 can be stored or used without regard to orientation, as the permeable bladder and intrinsic and extrinsic pressure on the bladder pushes evaporated gas within the bladder out of the permeable bladder liner such that primarily liquid fuel is contained in the bag, without a significant gas volume, and while maintaining uniform liquid fuel pressure for delivery. Hence, substantially liquid fuel is delivered through the fuel port without being interrupted by gas transfer without regard to orientation, thereby allowing the associated coupled fuel cell operation to be maintained continuously without regard to device orientation. In a preferred case, the coupling tube (not shown) extends inside the bladder approximately halfway to extract a suitable mixture of formic acid fuel. Cartridge 20 of
Portable fuel cells are often used to power mobile devices, and should preferably be small in size and integrated within handheld housings. In the case of cell phones, the handheld housing is small and held close to the users head. The cartridge is preferably plugged into the fuel cell ports and hot swappable. A problem is thus created of how to route and filter both fuel cell product exhaust and cartridge released gases within a confined space. A solution is to process the fuel cell system exhaust at the cartridge. To capture the formic acid vapor exiting the cartridge, a fuel cartridge 20 with integrated exhaust management (shown in
Passive Fuel Delivery Module
The fuel delivery module functions primarily to route the fuel and to control the fuel dose volume, and requires a fuel pressure differential between the fuel cell conduit and the bladder fuel pressure. As shown in
In the case where fuel cell module 60 includes two or more electrochemical fuel cells, as shown in
As shown in
Fuel Cell Module
Fuel cell module 60 includes one or more electrochemical fuel cells, shown in
Each of fuel cells 62a, 62b, 62c, 62d and 62e also includes a cathode, one of which is shown in
In each of fuel cells 62a, 62b, 62c, 62d and 62e, a cation exchange membrane, one of which is shown in
The passive pumping fuel cell system is operable with a wide range of fuel cell designs for local fuel distribution at the anode. It is generally preferred to have a uniform local fuel distribution, such that a fuel distribution layer uniformly and locally distributes fuel with the fuel cell without regard to orientation, and a reduction in effective fuel concentration at the anode surface such that a highly concentrated fuel can be used with high energy storage density. When the fuel cell module 60 of passive pumping fuel cell system 10 incorporates the referenced fuel distribution layer at the anode, the entire system 10 becomes orientation independent providing uniform fuel delivery and operation without regard to orientation, representing a substantial advance over known designs. Alternate local distribution layers can be substituted such as a fuel wick but are less preferred.
Exhaust Module
Exhaust module 80 includes an exhaust module inlet 81 for receiving consolidated fuel cell anode exhaust stream 67 and an exhaust module outlet 83 fluidly connected to fluid delivery module recycle liquid fuel stream inlet 53. A gas-liquid separator 82 is interposed between exhaust module inlet 81 and said exhaust module outlet 83.
Gas-liquid separator 82 includes a first chamber 82a and a second chamber 82b. First chamber 82a includes an inlet 85 for admitting anode exhaust stream 67 into first chamber 82a and an outlet 83 for discharging a recycle liquid fuel stream 87. Exhaust module 80 preferably includes a particulate filter 88 interposed in recycle liquid fuel stream 87 discharged from gas-liquid separator first chamber outlet 83. Second chamber 82b includes an outlet 93 for discharging a gaseous exhaust stream 89.
A gas-liquid separator membrane 82c is interposed between first chamber 82a and second chamber 82b of gas-liquid separator 82, and when completely blocked with liquid acts as a passive shutoff valve. Separator membrane 82c permits diffusion of at least a portion of the gaseous exhaust stream constituents present in anode exhaust stream 67, from first chamber 82a to second chamber 82b. Vapor permeable polytetrafluoroethylene liners can be used. Gaseous exhaust stream 89 is discharged from second chamber 82b. The gas-liquid separator is configured to provide the following functions for the case of formic acid fuel: from an intake of dilute formic acid and CO2, CO2 passes across the membrane, thereby creating a pressure differential that is proportional to fuel flow rate through the fuel cell anode passages. The dilute formic acid liquid is collected, without regard to system orientation, next to a drain trap check valve, which when open requires the liquid to escape before gas is recycled. Following liquid transfer to the bladder, gas is recycled back into the bladder, to the cartridge interior, thereby restoring cartridge pressure for fueling mode.
One or more vapor cells, which in system 10 of
An optional vapor cell 84 can be included in the exhaust module in series with the gas-liquid separator, to reduce fuel in the vapor from the separator. This is beneficial in the case of formic acid fuel, if the cartridge filter has saturated and reduced ability to filter the byproducts. The vapor cell 84 has a configuration that is substantially identical to fuel cells 62a, 62b, 62c, 62d and 62e, and includes an anode 84a, which is fluidly connected to gas-liquid separator second chamber outlet 93. Vapor cell anode 84a promotes electrocatalytic conversion of at least a portion of gaseous exhaust stream 89 to cations and a vapor cell anode exhaust stream 97. Vapor cell anode exhaust stream 97 includes un-reacted constituents from gaseous exhaust stream 89, if any, and vapor cell anode reaction product.
Vapor cell 84 also includes a cathode 84c for promoting electrocatalytic reaction of cations produced at vapor cell anode 84a with an oxidant stream (depicted as oxygen (O2) from air in
Moisture Management Module
Additionally, moisture management can optionally be included, if the application requires it. As shown in
An air plenum 106 in fluid contact with wick layer 102 directs an air stream over wick layer 102 such that at least some of the water generated at fuel cell cathode 64c and the other fuel cell cathodes, as well as at least some of the water generated at vapor cell cathode 84c is drawn away and evaporated into the air stream directed through plenum 106. A passive air filter 104 is preferably interposed between wick layer 102 and air plenum 106. As further shown in
Power Management Module
As further shown in
As further shown in
Power management module 100 can also include a cell voltage monitor electrically connected to and/or integral with microcontroller 122. The cell voltage monitor is capable of directing electrical signals to microcontroller 122 in response to voltage variations across fuel cells 62a, 62b, 62c, 62d and 62e. Microcontroller 122 is also capable of effectuating operational changes via electrical signals, one of which is depicted in
As illustrated in
System Operation
System 10 is especially well-suited to vaporizable liquid fuels capable of electrocatalytic conversion in direct liquid feed fuel cells. Preferred fuels include vaporizable liquid organic compositions capable of electrocatalytic conversion in direct liquid feed fuel cells, especially those in which vapor cell anode exhaust stream 97 contains carbon dioxide. System 10 is particularly well-suited to formic acid, more particularly an aqueous formic acid solution, which is a vaporizable liquid organic composition capable of electrocatalytic conversion to protons, carbon dioxide and water in anodes of direct liquid feed fuel cells. The present system enables recycling of un-reacted formic acid in liquid form back to the stored fuel, while vaporous fuel present in the anode exhaust stream is separated from liquid formic acid in a gas-liquid separator, and the vaporous fuel is then returned to the cartridge where it is filtered prior to exhaust as substantially benign carbon dioxide. Thus the formic acid fuel cycles within the closed system in liquid form, excepting where it is reacted or vapor byproducts filtered and exhausted. Passive circulation of fuel should maintain a pressure differential between the stored liquid fuel in the bladder and remaining fuel in the anode chamber of the fuel cell in series with the gas-liquid separator. The operation will be discussed with respect to these pressures.
Flexible bladder 22 contains a liquid fuel such as aqueous formic acid. Typically the bladder would start filled to its maximum expansion, resulting in about 90% of the cartridge interior volume. Inlet/outlet port 25 is fluidly connected to bladder 24. Port 25 intermittently discharges and admits a pressurized fuel stream 25 from and to bladder 24.
As further shown in
The cartridge illustrated in
Delivery of liquid fuel from the cartridge bladder 24 to the fuel cell anode 67 is enabled when the internal bladder fuel pressure P2 is greater than the fuel line pressure P3. The cartridge is pressurized with primarily CO2 when the stored fuel is formic acid fuel and inert gas such as air drawn in by the vacuum relief valve. Fuel is passively delivered from bladder to fuel cell, when the fuel valve 150 is opened by the controlled for a duration selected to provide a fuel dose to each fuel cell, appropriate for the cell volume and reaction rate necessary or desirable for the load, and can be customized based on cell voltage feedback. Fuel fills the cells saturating the anode fluidic reservoir described for the preferred case. The pressure differential can be controlled by lowering or relieving the current draw from the fuel cells, which decreases fuel cell line pressure as CO2 exhaust flow drops. Note this passive fueling only requires controlling a single valve with low parasitic power, and no pumps or powered actuators. It is instructive to review the change in pressure P2 with time. As described previously, when the liquid fuel is initially stored in the bladder, the compression elements 26a provide a minimal pressure differential for fueling, and as pressure P1 builds up within the housing, additional pressure contribution is added.
With respect to recycling there are three inter-related fluid transfers from the resulting fuel cell operation. First, separated exhaust gas is released through the cartridge filter and expelled. Second, excessive exhaust CO2 pressure is created when the gas-liquid separator is full and “shut-off” and the fuel cell is operated in high current mode, resulting in a backpressure flowing into the cartridge bladder which then migrates through the bladder liner to the interior cavity pressurizing the bladder. Third, unused liquid fuel is forced by the increased CO2 pressure to be pushed through the check valve 52 and into the cartridge bladder to maintain bladder volume.
Separated exhaust gas traverses conduit 89 through to fuel cell exhaust port 91 (and through optional vapor cell 84 if included). When the cartridge is coupled to the fuel cell system, port 91 is opened, releasing the exhaust gas to traverse a passage through to filter 30 where CO2 is removed and the remainder exhausted to ambient. The pressure differential in this case only has to be above ambient to exhaust the gas.
The method of returning gas to the bladder in the cartridge is described in
The third transfer mode can be termed liquid fuel recycling in that unused liquid fuel which is forced by the increased CO2 pressure to be pushed through the check valve 52 and into the cartridge bladder to maintain bladder volume, since it remains in the bladder, and the method is described in
The formic acid fuel stored in the bladder is diluted by the returned depleted fuel, however, many return fuel cycles can be performed to maintain passive fuel pumping, before the formic acid concentration (by weight) is reduced below a usable threshold. For example, the initial fuel can start at 70% by weight formic acid, and through multiple fuel returns can be reduced to 20% by weight formic acid, at which threshold the cartridge requires refueling. Alternatively the cartridge can optionally include a sensor(not shown) responsive to the formic acid concentration in the bladder, for example a visual indicator or chemical strip indicating when concentration is too low. Preferably the associated fuel cell system is discontinuously operable by the controller 122, to allow for switching between delivery and return conditions. A fuel cell system for discontinuous hybrid battery charging would be appropriate, as shown in
In operation, when flow regulating mechanism 150 is in an open position and the pressure in the bladder exceeds the fuel cell pressure, pressurized fuel stream is discharged from cartridge module 20 into fuel cell module 60. When flow regulating mechanism 150 is in a closed position, recycle fuel stream 37 can be returned into cartridge module 20, when the pressure differential P3>P2 exists. When gas-liquid separator is in “shut-off” mode, a sensor (not shown) can provide a signal to the flow-regulating mechanism to open to allow the backpressure of CO2 to enter the cartridge bladder, or alternatively the flow-regulating mechanism 150 can have an integrated check valve (not shown) to allow backpressure flow above a set-point pressure.
In operation of system 10 with the optional vapor cell, vaporous fuel in anode exhaust stream 89 is converted in vapor cell 84 to substantially benign vapor cell anode reaction product and un-reacted gaseous exhaust stream constituents, if any. Such un-reacted gaseous exhaust stream constituents are then directed through cartridge filter 30, where they are trapped and a benign exhaust stream is discharged from cartridge module 20. The optional vapor cell assists when the cartridge filter has expired or alternatively the rate of excess unused fuel exceeds the capability of the gas-liquid separator to process it and an optional bypass conduit (not shown) could be included.
The advantages of the present passive-pumping liquid feed fuel cell system include replacing wicking systems, active suction pumps, or bulky mechanical springs commonly used in delivering fuel from a cartridge, by utilizing the unique vaporizable properties of liquid fuels such as formic acid. Also, providing a closed liquid fuel circuit for storing and reusing unused fuel, by passively pumping unused fuel back into the cartridge. Providing a method of maintaining delivered fuel pressure differential in a precise and controlled range for micro-dosing fuel in low power applications, results in improved overall efficiency. The overall number and complexity of required components in the fuel cell system, by integrating functions between a cartridge and a fuel cell system. Most importantly, the embodiments described allow continuous operation of the fuel cell without regard to user orientation.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.
This application relates to and claims priority benefits from U.S. Provisional Patent Application Ser. No. 60/755,483, filed Dec. 30, 2005, entitled “Passive-Pumping Liquid Feed Fuel Cell System”. The '483 provisional application is hereby incorporated by reference herein in its entirety.
Number | Date | Country | |
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60755483 | Dec 2005 | US |